2006 Trailblazer 4.2L engine knock pressure transducer testing ?

Robert Diagnostician Florida Posted   Latest   Edited  
Pico Technology
2006 Chevrolet Trailblazer 4.2L (S LL8) 4-spd — 1GNES16S966119418
Engine Knock

Well its not likely we will tear into this motor but wanted to share what we found and what our guess is on the problem. Vehicle came in with what sounds like a lower end knocking / Bearing type sound. vehicle was run low on oil and added before it got to us. No codes stored other than a purge code when not commanded. Looked at the timing chain phasing with vehicle running looks good. Misfires show some misfires #5 cylinder especially when the knocking sound comes and goes. Decided to do some in cylinder testing ( still fairly new with this type of testing ) started with a known good cylinder #2 and #3 did cranking and running compression test and recorded those. Next check #5 cylinder since it was the one showing misfires. Found running and cranking compression both higher than cylinder #2 also notice the exhaust valve opening and intake valve opening shifted on #5. Looked at cylinder #1 not shown here and it was similar to #2. Thought here without further disassembly yet ( That may not happen unfortunately ) that the bearing may have spit on #5 cylinder causing piston to travel higher hence the higher compression ??? and not sure why the valve events slightly shifted other than the extra travel is effecting it. any insight would be helpful. The fourth capture over is cylinder #5 cranking i forgot to label that one.

Stephen Technician

From the two running captures; I think bank 2 cam is advanced. The scenario makes one normal 3 cylinder engine connected to the same crankshaft and sensors of another 3 cylinder engine that is out of time. I have seen another motor with that condition adjust the fuel to compensate for the bad side and set codes for the good side.

Ð Awarded
Ray Diagnostician

Is the 4.2 an in line 6 cylinder?

Ð Awarded
Geoff Diagnostician

Ray, yes it is.

Ð Awarded
Robert Diagnostician

This is an in-line 6 cylinder cam and crank relationships with zero advance was confirmed. There is a slight shift of exhaust valve opening on cylinder #5 compared to the two other cylinders I checked. I’m thinking that because the piston is going up slightly higher the extra peak times is actually what is causing that shift. I have looked at many scenarios using this type of testing good and…

Ð Awarded
Walter Manager

Those engines are famous for the lower end knock. When having been run low on oil or lack of oil changes they usually take out the main bearings first, but sometimes the rods. A weird aftereffect is you can remove the valve cover vent hose and plug the valve cover side with your finger or whatever and the knock will go away or become much quieter as it builds pressure in the crankcase. You will…

Ð Awarded
Matthew Mobile Technician

That engine does have some common lower end issues. The piston is still moving, air is going somewhere. When I have low running compression I use transducers in the intake, exhaust, and crankcase. Sync to your bad cylinder and look for a pressure pulse.

Ð Awarded